Charge detection for ink jet printers

ABSTRACT

Circuits are described involving a method of detecting charges on drops in an ink jet stream without contacting the stream. This permits a determination of proper synchronization and charging of the drops, the velocity of the ink stream, the extent of deflection of the drops, and whether or not the stream is in operation, or has stopped.

nited States Patent [191 Carmichael et a1.

Dec. 3, 1974 v CHARGE DETECTION FOR INK JET PRINTERS Inventors: John M. Carmichael; Donald L.

West; Robert A. Williams, Lexington, all of Ky.

[73] Assignee: international Business Machines Corporation, Armonk, NY.

Filed: Aug. 17, 1973 Appl. No.: 389,291

US. Cl. 346/75, 324/61 R, 324/72 Int. Cl. G0ld 18/00 Field of Search 346/75; 317/3; 324/61 S,

[56] References Cited UNITED STATES PATENTS 10/1973 Julisburger et a1. 346/75 OTHER PUBLICATIONS Ruddy, G. A.; Position And Synchronization Sensor For An lNR Jet Printer; IBM Tec. Disc. Bulletin, Vol. 15, No. 9, February 1973, pp. 2785-2786.

Naylor et al.; Differential Synchronization Sensor; IBM Tec. Disc. Bulletin, Vol. 16, No. 3, August 1973, pp. 776777.

Primary E.raminerJoseph W. Hartary Attorney, Agent, or FirmD. Kendall Cooper Circuits aredescribed involving a method of detecting charges on drops in an ink jet stream without contacting the stream. This permits a determination of proper synchronization and charging of the drops, the velocity of the ink stream, the extent of deflection of the drops, and whether or not the stream is in operation, or has stopped.

ABSTRACT 6 Claims, 5 Drawing Figures GUTTER PATENKLBEB wn 3,852,768

SHEET NF 2 5 fi FET l- GAIN THRESHOLD DETECT 10 COUPLED -15 fl-oSTR0BE CHARGE 2 +H.V. DEFLECTION ELECTRODE/ 50 PLATES a .6

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3.060: 7 i GUTTER T" I LE;

SHIELD H G 2 I 0/ FIGQ 3 THIN FILM M0005" v T3 )3 6 .000" yfib THIN FILM v -;0005 LJ 5C I .005 COPPER IL 003"MYLAR GUARD v .005" THICKNESS 5 .005" DIAMETER INSULATION FIG. 4

' THRESHOLD I 15 H 0 l h STROBE CHARGE DETECTION FOR INK JET PRINTERS BACKGROUNDOFINYENTION.FIELDAND PRIOR ART The present invention has particular utility in the field ofink jet printing. Various schemeshavebeen proposed heretofore for printing with drops of ink that are generated at extremelyhigh frequencies, such as in the range of I kilohert'z, or higher. In systems of this nature, such as that'set forth in the Woods, et al. application referred to above, it is extremely important that the drops of ink be checked in order to determine that their'velocity is correct, that their placement is correct, etc. An inductive type sensor is described in the US. Pat. application Ser. No. 313,913, having John Ghoug asian, et al. as inventors; filed-Dec. Li, 1972, and entitled Drop Charge Sensing Apparatus for An Ink Jet Printing System.

Also, another form of drop charge sensing .is described in the US. Pat. application Ser. No. 389,290 filed Aug. 17, I973, having Hugh E. Naylor, et al., as inventors, filed concurrently herewith, and entitled Deflection Sensors for Ink Jet Printers.

SUMMARY sensor is also coupled to a detection circuit for developing signals from drops passing through the sensor.

OBJECTS A prime object of the present'invention is to provide improved sensor arrangements for ink jet printers, the

sensors having greater efficiency, greater sensitivity, and improved signal to noise characteristics.

The foregoing and other objects, features, and advan tages of the invention will be apparent'from the following more particular description of various embodiments of the invention as illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS In the Drawings FIG. 1 illustrates a detection circuit incorporating a sensor. the sensor being shown in greater detail in FIGS. 2 and 3.

I FIG. 4 illustrates a variation further comprising a probe sensor having an external shield that is grounded.

FIG. Sis another variation using a planar sensor. DESCRIPTION Non-contact sensing of the charge on an ink'stream provides for fast charge synchronization in an ink jet printer when sensitive sensors and detection circuits are provided as shown in FIGS. 1 and 4.

The circuit shown in FIG. 1 permits the use ofa noncontact sensor shown in more detail in FIGS. 2 and 3 to sense the charge on afew drops or a single drop of ink. An ink stream [comprising a plurality of drops proceeds from left to right originating from a nozzle. such as that shown in the Woods, et al applicationf'The stream passes througha charge electrode 2 that places a variable charge on each drop of ink in accordance with' data signals, and the like. Just beyond charge electrode 2 is sensor 3 comprising a center conductor and an inner-shield 3b, togetherwith an outer shield 3;-

that -is grounded. The variably charged ink drops proceed between a pair of deflection plates 5 and as is known in the art are deflected in accordance with the amoun'tof charge placed thereon; Drops required for printing strike paper 6 while those not required are sent to a gutter8. A'Field Effect Transistor (FET) operational amplifier 10 connected as shown in FIGS. 1 and 4 to the center conductor 32 and the inner shield 31; of

sensor 3 effectively reduces the input capacitance of thecircuit to Zero. A large value resistor.l6 is connected as shown in FIGS. 1 and 4 to supply bias current to the FEToperational amplifier. This results in the ability to senseone drop of charged ink with a charge per drop less than 3 lO-l4 coulombs and with a frequency of less than 2500 samples/second. The output of amplifier 10 is AC coupled to another amplifier circuit l1 and thereafter to'a threshold circuit 12 with a signal ultimately provided on line 13. Such signal may be incorporated in various circuits to control synchronization in the system as discussed in the Woods, et al application.

As shown in FIGS. 2 and 3, the center conductor 3a and the inner shield 3b are protected by an outer (guard) shield 3c connected to ground to reduce the noise pickup of the sensor. In addition to the outer shield the threshold circuit is time strobed on line 15 to reduce the effects of any noise pickup. This strobing time isolates the main noise source which is the charge electrode, and the charge-sensor sense time, during which the charged drops are passing the sensor.

More details of the laminated sensor are shown in FIG. 3 wherein the layers of the sensor comprise alternate sheets of copper and Mylar* *Registered Trademark of Dupont). I

The configuration in FIG. 4 comprises a cylindrical member 3 with cylindrical elements 3a, 3b, and 3c serving functions similar to those previously described. This configuration is characterized as a probe-type sensor, rather than a laminar sensor.

The circuit in FIG. 1 and sensor configurations of FIGS. 2 and 3 or FIG. 4 can be used to detect the charge placed upon a drop of ink, thereby indicating when the charging time of the charge electrode is synchronized with the ink drop formation.

The configuration in FIGS. 50 and 5b represents a probe-type sensor 3 constructed on one plane, rather than as a cylinder like the sensor in FIG. 4. However, similar elements are similarly referenced. Additional applications: i

1. Velocity By placing two sensors along the stream of ink some distance apart,,then measuring-the time it takes a drop of charged ink to move the known distance, a velocity can be observed (v distance/time). A package or a single drop of charged drops can also be used'but with somewhat less resolution;

2. Deflection By placing one or more sensors at a position by which the drop ordrops of ink (charged ink) are to pass if properly charged and deflected. if proper deflection is not measured, then the various parameters of the system (ink stream'control parameterslmay be closed within the loop including the sensors.

. 3. Stream Running The stream of ink that is unused can be charged enough to keep it in the gutter but still have enough charge on the drops for the non-contact sensor and circuit to sense if thestream is runningJLoss of sensor output indicates stream failure,

While the invention has been particularly shown and describedwith reference to several embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.

What is claimed is:

l. A sensor system for ink jet printers, particularly providing reduced noise and'improved signal to noise ratio, comprising:

nozzle means for forming and propelling a stream of ink jet drops, charging means for charging said ink drops, deflecting means for deflecting said ink drops in accordance with the charge on said drops in a predetermined path toward a document to be printed, or the like; sensor means positioned downstream from said nozzle means in the path of travel of said ink drops,

said sensor means comprising an assembly of laminar elements including a sensor element, an inner shield, and an outer shield, and said sensor means having an aperture through which said ink drops pass, the arrangement being such that drops passing through said aperture are capacitively coupled to said sensor means for generating charges thereon in timed relation to passage of drops through the aperture of said sensor means; means interconnected with said outer shield for electrically grounding said outer shield to thereby reduce noise pickup by said sensor means; anda circuit interconnected with said sensor element and said inner shield for developing signals representative of detected charges. 2. The apparatus of claim 1, wherein said sensor means comprises:

alaminated structure composed of alternate layers of copper and Mylar having an aperture formed therein, the copper laminations serving as the sensor element, inner shield, and outer shield elements. 3. The apparatus of claim 1, wherein said sensor means comprises:

a cylindrical structure composed of concentrically mounted elements comprising a central concentric metallic sensor element, an inner shield surrounding said sensor element, an outer shield surrounding said inner shield, insulation interposed between said shields, and an aperture formed through the center of said sensor element in said cylindrical structure.

4. The apparatus of claim 1, further comprising:

a field effect transistor amplifier in said circuit means; and

means interconnecting said sensor element and said inner shield element to said amplifier for detection of charges developed in said sensor means.

- 5. The apparatus of claim 1, further comprising:

means positioning said sensor means relative to said charging means in order that charges originating from said charging means are developed in a balanced fashion on both said inner shield and said outer shield thereby having an equal and opposite effect, and cancelling out charges induced by said charging means in relation to charges induced by said individual ink drops as they pass through the aperture of said sensor means.

6. The apparatus of claim 1, further comprising:

strobe means incorporated in said circuit means for time strobing charges induced in said sensor means in order to further minimize effects of noise induced in said sensor means. 

1. A sensor system for ink jet printers, particularly providing reduced noise and improved signal to noise ratio, comprising: nozzle means for forming and propelling a stream of ink jet drops, charging means for charging said ink drops, deflecting means for deflecting said ink drops in accordance with the charge on said drops in a predetermined path toward a document to be printed, or the like; sensor means positioned downstream from said nozzle means in the path of travel of said ink drops, said sensor means comprising an assembly of laminar elements including a sensor element, an inner shield, and an outer shield, and said sensor means having an aperture through which said ink drops pass, the arrangement being such that drops passing through said aperture are capacitively coupled to said sensor means for generating charges thereon in timed relation to passage of drops through the aperture of said sensor means; means interconnected with said outer shield for electrically grounding said outer shield to thereby reduce noise pickup by said sensor means; and a circuit interconnected with said sensor element and said inner shield for developing signals representative of detected charges.
 2. The apparatus of claim 1, wherein said sensor means comprises: a laminated structure composed of alternate layers of copper and Mylar having an aperture formed therein, the copper laminations serving as the sensor element, inner shield, and outer shield elements.
 3. The apparatus of claim 1, wherein said sensor means comprises: a cylindrical structure composed of concentrically mounted elements comprising a central concentric metallic sensor element, an inner shield surrounding said sensor element, an outer shield surrounding said inner shield, insulation interposed between said shields, and an aperture formed through the center of said sensor element in said cylindrical structure.
 4. The apparatus of claim 1, further comprising: a field effect transistor amplifier in said circuit means; and means interconnecting said sensor element and said inner shield element to said amplifier for detection of charges developed in said sensor means.
 5. The apparatus of claim 1, further comprising: means positioning said sensor means relative to said charging means in order that charges originating from said charging means are developed in a balanced fashion on both said inner shield and said outer shield thereby having an equal and opposite effect, and cancelling out charges induced by said charging means in relation to charges induced by said individual ink drops as they pass through the aperture of said sensor means.
 6. The apparatus of claim 1, further comprising: strobe means incorporated in said circuit means for time strobing charges induced in said sensor means in order to further minimize effects of noise induced in said sensor means. 